FTIR spectroscopy is often used to identify unknown materials by comparing a sample spectrum to reference spectra. In many cases, this works well when the material is relatively simple and the sample condition is straightforward.
But in real manufacturing environments, materials do not exist as ideal reference samples. They are molded, stretched, annealed, welded, extruded, laminated, heat treated, and exposed to different cooling rates and stress histories. These processing steps may not change the basic chemical identity of the material, but they can still change how the FTIR spectrum looks.
This is an important distinction. A material can remain the same polymer or chemical system while showing noticeable spectral differences because of how it was processed.
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